Formatted question description: https://leetcode.ca/all/1116.html

# 1116. Print Zero Even Odd

Medium

## Description

Suppose you are given the following code:

class ZeroEvenOdd {
public ZeroEvenOdd(int n) { ... }      // constructor
public void zero(printNumber) { ... }  // only output 0's
public void even(printNumber) { ... }  // only output even numbers
public void odd(printNumber) { ... }   // only output odd numbers
}


The same instance of ZeroEvenOdd will be passed to three different threads:

1. Thread A will call zero() which should only output 0’s.
2. Thread B will call even() which should only ouput even numbers.
3. Thread C will call odd() which should only output odd numbers.

Each of the threads is given a printNumber method to output an integer. Modify the given program to output the series 010203040506... where the length of the series must be 2n.

Example 1:

Input: n = 2

Output: “0102”

Explanation: There are three threads being fired asynchronously. One of them calls zero(), the other calls even(), and the last one calls odd(). “0102” is the correct output.

Example 2:

Input: n = 5

Output: “0102030405”

## Solution

This problem can be solved using semaphores. In the class, create three semaphores semaphoreZero, semaphoreOdd and semaphoreEven for methods zero(), odd() and even() respectively. Initially, semaphoreZero has 1 permit, while semaphoreOdd and semaphoreEven has 0 permits.

For each method zero(), odd() and even(), in the loop, there are three steps. The first step is to acquire a permit from the current semaphore. The second step is to call the printNumber.accept(x) method for the current number. The third step is to release a permit back to the next semaphore.